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Project Notes

#295 FrequencyCounterKit

Build and test a common PIC-based frequency counter kit.

Build

Notes

I picked up a kit similar to this one after watching Julian Ilett’s videos on the kit build.

Not that I really need another frequency counter, but it is interesting to see how this one is constructed.

Julian’s investigation of the circuit was thorough, including the discovery that the kits available are deritative from original work by Wolfgang “Wolf” Büscher, DL4YHF, in his Frequency counter with a PIC and minimum hardware.

Specifications

From seller product information pages:

  • Color: Red, Simple Construction , compact design, easy to install and debug. Designed to measure the frequency of oscillation of most crystal oscillators. Based on PIC microcontroller can measure the frequency of 1 Hz to 50 MHz. Five digits of precision, for example kHz x.xxxx, MHz or xx.xxx x.xxxx MHz. General for feeding USB interface can be used for power supply, AC adapter or a battery of 9 V can be used too, it is used. Color: Red, Simple Construction , compact design, easy to install and debug.
  • Designed to measure the frequency of oscillation of most crystal oscillators.
  • Based on PIC microcontroller can measure the frequency of 1 Hz to 50 MHz.
  • Five digits of precision, for example kHz x.xxxx, MHz or xx.xxx x.xxxx MHz.
  • General for feeding USB interface can be used for power supply, AC adapter or a battery of 9 V can be used too, it is used.
  • Optional mode of power saving: can automatically convert the display if the frequency does not change significantly in 15 seconds
  • Very small number of components: a PIC16F628, 5, 7-segment LED screens, a 20 MHz crystal and some resistors
  • Frequency range: 1 Hz-50 MHz
  • Glass Test Range: 4 KHz-48 KHz
  • Built Item Size: 8 * 5.5 * 0.7 cm / 3.2 * 2.2 * 0.3 inches

Parts

Ref Item Qty
IC1 PIC16F628 1
IC2 7550A-1 5V regulator 1
LED1-5 5161AS 7-segment LED, common-cathode 5
D1-4 1N4148 diode 4
XTAL1 20MHz crystal 1
C1,3,4 22pF capacitor 3
C2 10-40pF variable capacitor 1
C5 102 1nF capacitor 1
C6 104 100nF capacitor 1
Q1 S9014 NPN transistor 1
Q2 S9018 transistor 1
R1,2 10kΩ resistor 2
R3-11 1kΩ resistor 9
R12 100kΩ resistor 1
J3 barrel jack 1
J1,2 3-pin headers 2
S1 pushbutton switch 1
  DIP socket 1
  PCB 1

kit-parts

The PCB is a nice 2-layer layout:

kit-pcb-front

kit-pcb-rear

Construction

The circuit in this kit is based on Construction - Variant 2 by Wolfgang “Wolf” Büscher, DL4YHF. Modifications:

  • addition of the crystal tester circuit
  • variable capacitor on one leg of the crystal
  • 5V regulator on the power supply

Here is my redrawing of the schematic used in the actual kit:

Schematic

Build

FrequencyCounterKit_build_rear

Performance

Julian and others have reported issues with the frequency counter (temperature effects; failure to register). However in my tests I didn’t see any of these problems. I tested crystals from 4MHz to 16MHz and got quite accurate results in all cases. Here’s an example of a 7MHz crystal under test:

test-crystal-7mhz

With a function generator, I also got accurate readings form 1kHz to 24MHz (the upper limit of the function generator). The input signal is a 0V to 5V square wave:

test-fgen-1khz

test-fgen-24mhz

Configuration

The press-button is used to access a control menu for the device. To get good results, I had to make two changes:

  • “Zero” - resets the frequency offset. When I first tried the device, it had a 5MHz offset which needed clearing.
  • “PSave” - enables/disables power save feature. When I first tried the device, power save was enabled, which causes the device to keep turning off (may be misleading and make you think it is not working).

Input Connections

Input connections are not fully documented. Here’s how they seem to work. The connector and pin identifications refer to the schematic:

Connector Pin Purpose
J1 1 crystal leg 1; ground
J1 2 no connection
J1 3 crystal leg 2
J2 1 frequency input; direct connect to PIC pin 3
J2 2 VIN / bypass power
J2 3 ground / bypass power
J3 1 VIN / power jack
J3 3 ground / power jack
J3 4 ground / bypass power

J2 and J3 and configured to allow by-pass power supply. J3 is the barrel jack - when power is plugged in here, it by-passes any power supply connected on J2. For my tests, I just had power connected on J2.

Next Steps and Improvements

One of the issues with the counter is that it requires an input signal that registers across 0-5V as it is fed as a direct input to the PIC. This can be problematic:

  • no over-voltage protection
  • unable to read small signals
  • AC-coupled signals may not have sufficient high-side amplitude

The preamplifier described by DL4YHF looks like an interesting addition.

So perhaps next things I’ll do with this kit:

  • add a pre-amplified stage, perhaps with over-voltage protection
  • mount in a case with a BNC connector for input frequency
  • add a switch and battery connected on J2 (so can be mounted with a battery in a case, with external power bypass on J3)

Credits and References

About LEAP#295 PICToolsTest EquipmentOscillators
Project Source on GitHub Return to the LEAP Catalog

This page is a web-friendly rendering of my project notes shared in the LEAP GitHub repository.

LEAP is my personal collection of electronics projects, usually involving an Arduino or other microprocessor in one way or another. Some are full-blown projects, while many are trivial breadboard experiments, intended to learn and explore something interesting (IMHO!).

The projects are usually inspired by things found wild on the net, or ideas from the sources such as:

Feel free to borrow liberally, and if you spot any issues do let me know. See the individual projects for credits where due. There are even now a few projects contributed by others - send your own over in a pull request if you would also like to add to this collection.